A startup combustor is sometimes used in order to warm up a reformer of a fuel reforming system. The startup combustor preferably combusts fuel at approximately a stoichiometric ratio due to the large amount of produced heat. However since the combustion temperature is high when combustion operations are performed at approximately a stoichiometric ratio, NOx is produced largely as a result of reactions between oxygen and nitrogen in the air. Consequently there is the possibility that NOx components will be present in exhaust emissions from the fuel reforming system. Furthermore when the fuel reforming system is applied to a fuel cell, entry of NOx into the fuel cell constitutes a cause of deterioration or contamination of the fuel cell. One method of preventing the production of NOx comprises control of the combustion temperature in the combustor to a temperature at which NOx is not produced.
A conventional technique disclosed in Tokkai Hei 9-063619 published by the Japanese Patent Office in 1997 sets the excess-air factor (air-fuel ratio/stoichiometric ratio) of the startup combustor to a value greater than one, for example to a value of three. Consequently the combustion temperature is suppressed because of the excess air amount introduced into the startup combustor.